1. Field of the Invention
This invention relates to the field of homogeneous single crystal superalloy articles.
2. Description of the Prior Art
The nickel base superalloy art area has been extensively investigated for many years, and as a result there are very many issued patents in this area. Some of these disclose alloys in which no intentional additions of cobalt, carbon, boron, or zirconium are made, or alloys in which these elements are optional. These include for example U.S. Pat. Nos. 2,621,122; 2,781,264; 2,912,323; 2,994,605; 3,046,108; 3,166,412; 3,188,204; 3,287,110; 3,304,176; and 3,322,534. These patents do not discuss single crystal applications.
U.S. Pat. No. 3,494,709, assigned to the assignee of the present invention, discloses the use of single crystal articles in gas turbine engines. This patent discusses the desirability of limiting certain elements such as boron and zirconium to low levels.
The limitation of carbon to low levels in single crystal superalloy articles is discussed in U.S. Pat. No. 3,567,526 which is also assigned to the present assignee.
U.S. Pat. No. 3,915,761, assigned to the present assignee discloses a nickel base superalloy article produced by a method which provides a hyperfine dendritic structure. As a result of the fineness of the structure, the article may be homogenized in relatively short times.
The conventional nickel base superalloys which are used to fabricate such parts have evolved over the last 30 years. Typically these alloys contain chromium to levels of about 10% primarily for oxidation resistance, aluminum and titanium in combined levels of about 5% for the formation of the strengthening gamma prime phase and refractory metals such as tungsten, molybdenum, tantalum and columbium in levels of about 5% as solid solution strengtheners. Virtually all nickel base superalloys also contain cobalt in levels of about 10%, and carbon in levels of about 0.1% which acts as a grain boundary strengthener and forms carbides which strengthen the alloy. Boron and zirconium are also often added in small amounts as grain boundary strengtheners.
Most commonly, gas turbine blades are formed by casting and the casting process most often utilized produces parts having equiaxed nonoriented grains. It is well known that the high temperature properties of metals are usually quite dependent upon grain boundary properties, consequently efforts have been made to strengthen such boundaries (for example by the additions discussed previously), or to reduce or eliminate the grain boundaries transverse to the major stress axis of the part. One method of eliminating such transverse boundaries is termed directional solidification and is described in U.S. Pat. No. 3,260,505. The effect of directional solidification is to produce an oriented microstructure of columnar grains whose major axis is parallel to the stress axis of the part and which has minimal or no grain boundaries perpendicular to the stress axis of the part. A further extension of this concept is the utilization of single crystal parts in gas turbine blades. This concept is described in U.S. Pat. No. 3,494,709. The obvious advantage of the single crystal blade is the complete absence of grain boundaries. In single crystals, therefore, grain boundaries are eliminated as potential weaknesses, hence, the mechanical properties of the single crystal are completely dependent upon the inherent mechanical properties of the material.
In the prior art alloy development much effort was devoted to the solution of problems resulting from grain boundaries, through the addition of elements such as carbon, boron, and zirconium. Another problem which prior art alloy development sought to avoid was the development of deleterious phases after long term exposures at elevated temperatures (i.e. alloy instability). These phases are of two general types. One, such as sigma, is undesirable because of its brittle nature while the other, such as mu, is undesirable because the phase ties up large amounts of the refractory solid solution strengtheners thus weakening the remaining alloy phases. These phases have been termed TCP phases for topologically close packed phases, and one of their common properties is that they all contain cobalt. There are of course TCP phases which can form in the absence of cobalt but these cobalt free TCP phases contain other elements such as silicon which are not commonly found in nickel base superalloys. While an obvious remedy to control these deleterious phases is the removal of cobalt, this has not proved practical in prior art alloys for polycrystalline applications. The problem is that if the cobalt is removed the carbon combines preferentially with the refractory metals to form M.sub.6 C carbides which are deleterious to the properties of the material as their formation depletes the alloy of the strengthening refractory elements.
U.S. Pat. No. 3,567,526 teaches that carbon can be completely removed from single crystal superalloy articles and that such removal improves fatigue properties.
In single crystal articles which are free from carbon there are two important strengthening mechanisms. The most important strengthening mechanism is the intermetallic gamma prime phase, Ni.sub.3 (Al, Ti). In modern nickel base superalloys the gamma prime phase may occur in quantities as great as 60 volume percent. The second strengthening mechanism is the solid solution strengthening which is produced by the presence of the refractory metals such as tungsten and molybdenum in the nickel solid solution matrix. For a constant volume fraction of gamma prime, considerable variations in the strengthening effect of this volume fraction of gamma prime may be obtained by varying the size and morphology of the gamma prime precipitate particles. The gamma prime phase is characterized by having a solvus temperature above which the phase dissolves into the matrix. In many cast alloys, however, the gamma prime solvus temperature is in fact above the incipient melting temperature so that it is not possible to effectively solutionize the gamma prime phase. Solutionizing of the gamma prime is the only practical way in which the morphology of the gamma prime can be modified, hence for many commercial nickel base superalloys the gamma prime morphology is limited to the morphology which resulted from the original casting process. The other strengthening mechanism, solid solution strengthening, is most effective when the solid solution strengthening elements are uniformly distributed throughout the nickel solid solution matrix. Again this strengthening is reduced in effectiveness because of the nature of the casting process. Practical nickel base superalloys freeze over a wide temperature range. The freezing or solidification process involves the formation of high melting point dendrites followed by the subsequent freezing of the lower temperature melting interdendritic fluid. This solidification process leads to significant compositional inhomogenities throughout the microstructure. It is theoretically possible to homogenize such a microstructure by heating at elevated temperatures to permit diffusion to occur, however, in practical nickel base superalloys the maximum homogenization temperature, which is limited by the incipient melting temperature, is too low to permit significant homogenization in practical time intervals.